How do chromosomes and the environment contribute to human variation?
In addition to the genetic conditions caused by a single gene, there are other genetic conditions caused by the interaction of several genes that are considered diseases, disorders, or defects.
What Do You Think?
Insurance companies compensate people for medical expenses. Should they be allowed to screen people for genetic disorders before agreeing to insure them?
Conditions Caused by Several Genes
Some genetic conditions resulting in physical abnormalities are caused, not by a single gene, but by the interaction of several genes. Because several genes cause the condition, determining the chance of the condition being passed from a parent to a child is more complicated than for single gene defects. Some of the genetic conditions caused by the interaction of several genes include the following.
- Cleft lip, in which the person is born with a deformed upper lip
- Club foot, a condition in which the foot is turned in
Some genetic conditions are caused not by a single gene or the interaction of several genes, but by a chromosomal error. Sometimes chromosomes break during meiosis and a person is born with a piece of chromosome missing. Sometimes, during meiosis, the chromosomes do not separate properly, and a person is born with 47 chromosomes, instead of 46. The best known example of this kind of chromosomal condition is Down syndrome. People with Down syndrome have 3 copies of chromosome 21 instead of only 2 copies. They usually have an extra fold of skin near the eye, a large tongue, and small hands with short fingers. In addition, they may have defects of the heart, eyes, and ears. They also tend to have learning disabilities of various degrees of severity. Just as there is great diversity among people without Down syndrome, there also is great diversity among people who have Down syndrome. Some individuals with Down syndrome can function in society with reasonable independence, while others require a good deal of help and guidance.
Did You Know?
Today you are a different person than you were yesterday. Millions of cells in your body are dividing right now as you read, and mutations or errors are a natural part of DNA replication. Most of these errors have no adverse effect on you. Most of these DNA errors are repaired by our DNA repair enzymes. Some of these errors are harmless even if they are not repaired, but some can be deadly! Some DNA mutations can cause cancer, a condition of uncontrolled cell growth.
Did You Know?
Both genetic predisposition and environmental factors can influence your probability of getting cancer. Exposure to chemicals and radiation in the environment can damage DNA and cause cancer. Someone who has a less efficient DNA repair mechanism may be more susceptible to those environmental hazards.
Genetic Disease and Environmental Factors
There are a number of common diseases that can be caused by both the environment and the genes you are born with. Cancer and heart disease are two good examples. If you have a parent who had cancer, you are predisposed (more likely) to get cancer yourself. However, these conditions are so closely linked to environmental influences that it is often hard to tell the real cause. For example, suppose a parent dies from lung cancer (abnormal cell growth in the lungs) and smoked most of his or her life. It is hard to determine if that person developed lung cancer because his or her lungs were particularly sensitive to smoking, or if he or she would have developed lung cancer anyway as a result of heredity. Why do some people get lung cancer from smoking, while others do not?
If a parent has a disease, it is important for you to learn about the nature of that disease and the risk factors that might make it more likely that you, too, will get the disease. Risk factors are the activities and lifestyles you choose that make it more or less likely for an outcome to occur. For example, if you choose to be an emergency room physician, you will work in a stressful profession. You may be more likely to develop stress-related problems such as high blood pressure. If your family has a history of heart disease and stroke, a stressful profession might be a risk factor you should minimize. Maybe it would be better for you to become a dermatologist or a pediatrician. Other risk factors for heart disease are smoking, lack of exercise, and a high-fat diet. Everyone should minimize such risk factors. However, it is especially important for those individuals who have a genetic predisposition for developing heart disease to do so.
How do you know if you have a gene that puts you at risk for a disease? Genetic counseling is a profession that is mostly concerned with helping people who may have genetic-related conditions. It is estimated that 20% of all people have a genetic condition, have someone in their immediate family (mother, father, brother, sister, aunt, uncle, or cousin) with a genetic condition, or carry the allele for a defect that may be passed onto their children.
Genetic counseling is performed by a team of genetic specialists. This team might include the genetic counselor, a physician, the cytotechnologist (who makes the karyotype), laboratory technicians, and a molecular geneticist who is concerned about the structure and function of DNA. The genetic counselor is the person who coordinates the team and talks directly to the patients.
People see a genetic counselor for one of two reasons. Either they have given birth to a child with a genetic condition or they think they are at risk of giving birth to a child with a genetic condition, because of a family history of the disorder. Families who have given birth to a child with a genetic condition usually ask three questions.
- Why has this happened to us?
- What can we do about it?
- Will it happen again?
People who think they are at risk for giving birth to a child with a genetic condition want to know what that risk is. In both cases, the genetic counselor helps the family learn more about the condition.
What Do You Think?
If you carded a recessive gene for a genetic disorder, and your spouse did too, would you risk having children? Why or why not?
The first thing that a genetic counselor does when working with a family is gather all the information about the traits and variations of the family members to construct a pedigree chart. The genetic counselor will schedule meetings with the geneticist, physicians, and laboratory technicians for the family. The genetic counselor will help the family learn what is known about the condition. The genetic counselor may put the family in contact with other people who have the same genetic condition, so they can provide help and support for one another. There may be a genetic counselor in your community who will come to your class and talk about his or her work.
In studying genetics, we have considered some of the problems about how traits and variations are passed from parents to children so that there is both continuity and diversity within a species. The first people to study genetics were concerned about the relationship between allele pairs and variations. Molecular geneticists are concerned with problems involving the structure and function of DNA and how it directs the production of proteins in a cell. Genetic counselors are concerned about the health care of families with genetic conditions.
One group of geneticists that we have not discussed are called population geneticists. Population geneticists are concerned, not about a single family, but about how and why some alleles are found in people in certain parts of the world and not others. They wonder, for example, why many people in Central Asia have Type B blood, but very few American Indians have this blood type. By studying the genes in a population, population geneticists form a link to those scientists who study evolution. Population geneticists are trying to learn more about how living organisms, including humans, change over time.
Activity 8-1: Investigating the Human Genome Project
The Human Genome Project is an international effort to map and sequence all the estimated 80,000 to 100,000 genes in human DNA and to determine and analyze the sequence of the 3 billion (3,000,000,000) chemical bases that make up human DNA. The implications and applications of information gained through this project have already affected human lives. In the next section you will learn more about the Human Genome Project. This activity helps you obtain information about human genes and genetic disorders from the Human Genome Project.
- Human Genome Project address
- Computer with an Internet connection
- Letter paper, envelope, and stamp
- Presentation materials
Step 1 Select a number from 1 to 23. This number represents the chromosome you will investigate.
Step 2 Contribute to a class letter to the Human Genome Project Center requesting information about your selected chromosome. In the class letter, request information such as the size of the chromosome, the names and locations on the selected chromosome of the genes identified to date, and what characteristics the identified genes control.
Step 3 When your Human Genome Project information arrives, select one of the genes on your chromosome and investigate the trait (genetic expression) of the gene. If the gene you select has a known disorder, focus your research and presentation on the diagnosis, symptoms, and treatment of the condition.
Step 4 Prepare a presentation to your class about the gene you selected. Your presentation might take the form of a poster presentation, computer presentation, story, or a presentation format of your own choice.
- What is the difference between a multiple gene disorder and a chromosomal disorder?
- What are some risk factors for acquiring certain diseases, such as cancer or heart disease?
- Why is genetic counseling useful?
- What do population geneticists do?